Male reproductive function and sex characteristics are controlled by the hormones
of the hypothalamic-pituitary-gonadal axis. Gonadotropin-releasing hormone is synthesized
in the hypothalamus and is transmitted to the anterior pituitary gland where it stimulates
the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). LH
stimulates the interstitial Leydig cells in the testes to stimulate the conversion
of cholesterol through a series of chemical reactions to testosterone. Testosterone
production increases at puberty and is responsible for the secondary sex characteristics,
such as deepening of the voice, increase in muscle mass, and libido. Testosterone
circulates in the plasma in the free form but also is bound to proteins, such as albumin
and sex hormone–binding globulin (SHBG).
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